The Background section of this document is provided to place embodiments herein in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.
Many implementations of Fifth Generation (5G) base stations are expected to utilize so-called analog beamforming. This is due to the higher complexity, mostly from a hardware perspective, of implementing so-called digital beamforming. The latter imposes fewer functionality restrictions but is rather more costly to realize.
As used herein, “beamforming” means that a transmitter can amplify transmitted signals in selected directions, while weakening them in others. Correspondingly, a receiver can amplify signals received from selected directions while weakening unwanted signals in other directions. Analog beamforming in this context means that this can only be applied to one direction or a limited set of directions at a time by each transmitter/receiver. An array of multiple transmit antennas or receive antennas must be used to transmit or receive in multiple directions at the same time. To beamform, a signal is transmitted from multiple transmit antennas, but with individually adjusted phase shifts or time delays, which effectively creates a beam in the resulting transmit radiation pattern of the signal—e.g., through controlled constructive and destructive interference of the phase-shifted signals from individual antenna elements. The beam direction depends on the phase shifts of the antenna elements. Similarly, in the case of a receiver, phase shifts between antenna elements can be used to steer the maximal antenna sensitivity toward a desired direction.
Beamforming allows the received signal to be stronger for an individual connection, thereby enhancing throughput and coverage for that connection. It also enables a reduction in the interference from unwanted signals, thereby enabling several simultaneous transmissions over multiple individual connections using the same time-frequency resource, so called spatial multiplexing or Multiple Input Multiple Output (MIMO) using either a single user (SU)-MIMO, or multiple users (MU)-MIMO.
An important problem with beamforming is to decide which beam(s), i.e., which direction(s), to use for transmission and/or reception. To support base station beamforming, i.e., beamforming at a network node such as a base station, a number of reference signals may be transmitted in different beam directions, respectively, from the network node or base station. Each wireless device or User Equipment (UE) can measure these reference signals and report the measurement results to the network node. The network node may then use these measurement results to decide which beam(s) to use for data transmission to one or more wireless devices. As further described herein, a network node can use a combination of persistent and dynamic reference signals for this purpose.
The persistent reference signals, denoted herein as beam reference signals (BRS), are transmitted repeatedly in a large number of different beam directions. This allows a wireless device to measure the BRS when transmitted in different beams, without any special arrangement or instruction for that wireless device from perspective of the network node. The wireless device reports the received powers for different BRS back to the network node, along with an index of the BRS, given for example by the BRS sequence and the time and frequency position of the particular BRS. By reporting a BRS index and an associated received power of that BRS, the wireless device is effectively reporting its preferred beam. The wireless device may report a list of BRS indices and associated received powers, for example, its top eight strongest BRSs.
The network node may then transmit dedicated reference signals to a particular wireless device, using one or more beams or beam directions that were reported as strong for that wireless device. These are dedicated reference signals and may thus only be present when the wireless device has data to receive, and they give more detailed feedback information of the beam-formed channel, such as co-phasing information of the polarizations and the recommended transport block size, as well as the transmission rank in case of spatial multiplexing. Since the BRS is transmitted repeatedly over a large number of beams, the repetition period should be relatively long, to avoid using too much resource overhead for the BRS transmissions.
The dynamic reference signals, denoted herein as channel-state information reference signals (CSI-RS), are transmitted only when needed for a particular connection. The decisions of when and how to transmit the CSI-RS are made by the network node and signaled to the involved wireless devices using a measurement grant or configuration message. When the wireless device receives a measurement grant it measures on a corresponding CSI-RS. The network node can choose to transmit CSI-RS to a wireless device using only beam(s) that are known to be strong for that wireless device, to allow the wireless device to report more detailed information about those beams. Alternatively, the network node can choose to transmit CSI-RS also using beam(s) that are not known to be strong for that wireless device, for instance to enable fast detection of new beam(s) in case the wireless device is moving.
The 5G network nodes transmit other reference signals as well. For example, the network nodes may transmit a demodulation reference signal (DMRS) when transmitting control information or data to a wireless device. Such transmissions are typically made using beam(s) that are known to be strong for that wireless device.
In Fourth Generation (4G) systems, discovery reference signals (DRS) may be used for the same purpose as BRS, as described above. Hence, the LTE wireless device is configured to perform received power measurements on a set of different DRS signals and report the associated DRS index and measured received power for the eight DRS measurements with highest power. Accordingly, although described in the context of 5G, the principles and concepts discussed herein are applicable to 4G systems as well.
Beamforming is not restricted to network nodes. It can also be implemented in the receiver of the wireless device, further enhancing the received signal and suppressing interfering signals. The wireless device may also implement transmit beamforming. Similar to a network node, analog beamforming can be used in the wireless device, which means that the wireless device can only receive/transmit from/to one direction at a time, unless multiple receivers/transmitters are available.
When operating with the 5G base stations or network nodes, a wireless device with analog receive beamforming can measure the BRS using different device beams of the wireless device, and then choose the device beam(s) that provides the highest Beam Reference Symbol Received Power (BRSRP). However, care must be taken when comparing the Reference Signal Received Power (RSRP) of different device beams since the received power depends on the utilized combination of network and device beams. A given device beam may have a high BRSRP when paired with a certain network beam, but have a low BRSRP in combination with other network beams. A different device beam may also give an equally high BRSRP when combined with a different network beam, but give a low BRSRP in combination with all other network beams.
Since the network node may not transmit all BRS at the same time, e.g., due to limitations imposed by analog beamforming, but rather cycle through all network beams during some time window, it is important that BRSRP-values for different device beams that are compared stem from measuring the same network beam, otherwise the measurements may not be comparable.
Known implementations of beamforming in wireless communication networks do not provide mechanisms for robust management of network and device beams. Relying on the known solutions, device beam selection can only be done slowly, such that it can be treated as changes in the radio channel in order to keep it transparent to the network operation. This may result in that the beams are used in a non-optimal manner reducing or limiting the performance of the wireless communication network.